This invention generally relates to the transmission of messages between remote locations and more specificially to the transmission of such messages by means of a modulated carrier wave.
There are several well-known methods and apparatus for transmitting messages, especially messages in digital form, between two locations. In data processing systems, for example, such messages are transferred over conventional or specially adapted telephone lines, over microwave transmission systems and even by means of satellite communications systems. Each system, however, requires a substantial investment at each location. Thus, these systems link geographical centers of some significant population on the supposition that a sufficient number of customers will use the system and justify its expense.
When it is desirable to transmit a message to a geographically remote, sparsely populated location, it is not possible to justify these systems. In such situations, radio systems are used subject to certain operating limitations. Although they are relatively inexpensive systems, various disadvantages detract from their universal acceptance as communications systems in these applications. For example, anyone can copy a message even though that person is not intended to receive the message. Although message scrambling can overcome this problem, the overall expense of the system increases.
When a receiver responds to a carrier wave which is propagated by means of a ground wave or is received along a line of sight, the received signal is very reliable. Unfortunately, these propagation paths are effective only for limited distances. If a message must be transmitted over a greater distance, such a radio system must rely on a propagation medium which includes the ionosphere. However, this medium is very noisy under disturbed conditions which exist during auroras and magnetic storms. Moreover, the reflective characteristics of the ionosphere constantly change as the height of the ionosphere and its critical frequency vary. The critical frequency is that frequency above which a carrier wave does not reflect from the ionosphere. With a fixed transmitting station, it is not possible to predict where a reflected carrier wave of a given frequency will intercept the surface of the earth, when it will arrive or even if the ionosphere will reflect the carrier wave at all. Thus, even though such radio systems might appear to be attractive economically, they are not widely used to obtain reliable communications between fixed locations at any time.
Other communications media are also subject to the same or similar conditions which detract from their use as reliable propagation media for carrier waves. Such media include, by way of example, the troposphere, power cables and power lines and acoustical underground paths. All these media can be classified as highly noisy, variable delay propagation media.
Therefore, it is an object of this invention to provide a method and apparatus for enabling the reliable transmission of messages by means of such highly noisy, variable delay propagation media.
Another object of this invention is to provide a reliable message transmitting method and apparatus for highly noisy, variable delay propagation media which is economical to implement.
Yet another object of this invention is to provide an economical method and apparatus for transmitting messages using the ionosphere as the propagation medium.